Seismic surveying



June 27, 1944. J, Q PARR, JR 2,352,494

SESMIC SURVEYING Filed July 2l. 1942 JOSEP/706 Q @Flin/?,

@3% ummm 862g @nimm s Patented June 27, 1944 SEISMIC SURVEYING Josephus 0. Parr, Jr., San Antonio, Tex., assignor to olive s. Peay,

San Antonio, Tex.

Application `uly 21, 1942, Serial No. 451,779

4 Claims. This invention relates to methods of and apparatus for conducting seismic surveys for use in" connection with geophysical prospecting, and more particularly to apparatus and methods for recording the seismic wave trains as they arrive at one or more reception points.

It is a general object of the present invention to provide novel and improved apparatus for and methods of recording the arrival times of waves which are received over a period of several seconds and exhibit during 'that time various intensities or amplitudes, the ratio of the highest to the lowest amplitude being relatively large.

Thus it is well recognized that certain higher energy waves may be received with an intensity of the order of 600 times as great as certain other waves, and itis also noted that during a period following the reception of waves of high amplitude, the wave form energy is gradually attenuated over an appreciable period, usually decayingrather uniformly with time. Thus in order that waves of widely varying amplitude may all bev received and recorded by sensitive apparatus on a record sheet or other medium of convenient dimensions, it is highly essential that some means be provided to bring the waves, or the energy derived therefrom, to levels not greatly diifering throughout the total time of wave reception.

It has been proposed heretofore to provide seismic apparatus with means functioning automatically to regulate and control the amplitude of the recorded signal for the purpose of compensating, at least in part, for the wide variation in amplitude of the seismic wave trains. One such arrangement is described in the application for U. S. Letters Patent by Olive S. Petty, Serial No..290,9,28, led August 18, 1939. Thus that 'application discloses, in'association withY a seismometer or other device for converting seismic impulses into electrical wave form signal energy, an amplifier for such signal energy, the degree of amplification effected .in such amplifier being automatically regulated in response to change in amplitude of the seismic waves. In automatic volume controls of this character it is a common expedient to provide means whereby control4 is not effected unless and until the 'amplitude of the incoming signal exceeds a predetermined value, commonly called the threshold value, and in said prior application this threshold value is substantially constant during the taking of a. record.

. I have found, however, that itis highly advantageous to amplify the incoming signals to a greater extent toward the end of the record than in the earlier part of therecord, and while the circuit disclosed in said prior application -will -function satisfactorily for lthis purpose in the eventthe threshold value is exceeded and the .not likely to be confusing.

automatic volume control caused to ,function only once 'during th taking of the record, namely during the arrival of the early high energy waves, subsequent arrival of signals exceeding the threshold value in amplitude and causing repeated functioning of the A. V. C., nullifles to a large extent the effect of the circuit in affording gradually increasing sensitivity with time.

In accordance with the instant invention it is therefore proposed to vary the threshold value during the taking of a record, for example by effecting a gradual increase of this value with time, so that toward the end of the record the A. V. C. will be caused to function only on the arrival of seismic waves of substantially increased amplitude.

The desirability of increasing the amplifica.- tion factor or sensitivity of the amplifier toward the end of the record is particularly important.

because, in general, seismic waves reflected from deeper beds tend to be lower in frequency as well as in amplitude. Thus in the early portion of the record, the high frequencies predominate, and since the peaks of separate cycles on the recorded traces are quite close together, increase in amplification makes it difficult to determine the precise instant of arrival of the waves reflected from a given bed, especially if the ampliilcation is sufficient to cause overlapping of adjacent record traces. Toward the end of the record,- however, .where the peaks are fairly widely separated, increased amplification renders the arrival times much more readily apparent. Again, the step-out or displacement from one recorded trace to the next, assuming that the dip of the deeper beds is the same as that of the shallow beds, is much less toward the end of the record, and overlapping of adjacent traces is Consequently, an amplitude toward the end of the record of two or three times that at the rst portion of the record can be employed without any apparent crowding of the record and with a substantial improvement in interpretive clarity.

It is therefore an object of the invention to provide, in an A. V. C. circuit for amplification of' seismic energy. means whereby substantially greater amplification toward the end of the record than at the earlier part of the record occurs, regardless of the repeated arrival of impulses of substantial amplitude. More specifically, it is the object of the invention to provide, in apparatus 'of the character described, an automatic volume control in which the threshold value for its operation is gradually increased with time, preferably beginning with the time of arrival of the first high energy waves.

Further objectsand features of the invention will be apparent from the following description third stage amplifier valve` 49.

taken in connection with the accompanying drawing, in which the single figure is a circuit diagram of an amplifier for wave-form electrical signals illustrating one method of applying the i instant invention.

In order to facilitate an understanding Aof the invention, reference will be made to the embodiment thereof illustrated in the accompanying drawing and specific language will be employed.

output circuit which includes the -primary 26 of a transformer, f the secondary 21 of which is shunted by a pair of variable resistors 28 and 29,

onefor coarse and one for fine adjustment, for

manually setting the level of the signal'applied to the amplifier.

The secondary 21 feeds into the cathode and grid of the thermionic valve 30 which constitutes a simple triode amplifier having the anode 3|,

grid 32, and heated cathode 33 which it shares in4 common with the elements ofthe second stage amplifier valve indicated at 34. A suitable biasing battery 35 is arranged in the grid circuit of the triode amplifier, This amplifier is coupled to .the second stage amplifier by means of an anode coupling resistor 36 and condenser 31. The anode 3| as well as the anode 38 of the -second stage to assist in the desired lter action. At the same time, this condenser performs the vimportant function of assisting in the damping of the moving element of the galvanometer.

Inthe same envelope with valve 49 is a trlode, which may for convenience be referred to as the A. V. C. valve, comprising anode 61, grid 68, and

valve receive their-voltage supply through the vting more gain and improved automatic volume control. The control grid 43 of the second stage amplifier is coupled by the condenser 31 to the anode of the first stage and receives its bias through resistor 45 from a tap on battery 46. An adjustable coupling condenser 41 and anode resistor '50 provide the coupling between the second stage anode 38 and the control grid 48 of 'the Condenser 41 acts in the nature of a filter, first to avoid flow of anode current through the primary winding ,52 of the interstage coupling transformer 53, thus increasing the transformer life and permitting its design to be more ecient, and second,

to vary the frequency response ofthe amplifier,

giving it the highest gain on the desired frequencies, which effect comes from resonating the transformer primary.

The secondary winding 56 of the interstage transformer 53 connects between the control grid 48 and the bias battery. 46. The output from the.

third stage is fed through the primary 58 of transformer 59, one secondary 60 of which delivers the amplified waves to the moving element of a galvanometer associated with and forming part of a recorder 6 I. Another secondary 62 takes the common heated cathode 69 Which functions also with the elements of the third stage amplifier. The anode 61 is connected by wire 10 to a separate source 1| of anode voltage, the negative pole of which leads through variable resistors 12, 13, and 14 back to Wire 15 connected to the negative end of the source of heater current and common ground terminal. It will be noted that bias battery 46 is connected by wire 16 to the slider' of the resistor13 and hence is also con-` nected tothe ground through resistor 14 which is arranged in shunt with condenser 93. The battery 1| may have a potential of about 45 volts for the type of valve shown, while the value of resistors 12, 13, and 14 may be of the order of 200,000 ohms each.` A condenser 9| is connected across resistors 13 and 14 to ground.

The grid 68 of the A. V. C.` valve is fed from the secondary 62 of the output transformer 59, the opposite end of which is connected by conductor. 11 to the slider of a potentiometer 18 energized from battery 80, through the contacts 8| of a relay, the winding 82 of which is in series with the heaters of valves 30 and 49, the positive terminal of battery'being returned to the cathode of the A. V. C. valve through resistor |08.

The slider on the potentiometer 18 is set t0 Y* the .flow of any anode current in that valve.

Preferably the Valveis thus biased substantially `beyond cut-off, and flow of anode current occurs only during positive swings of the output -signal of sufficient magnitude to drive the grid across the cut-off point toward the positive side. Thus l Whenever the positive potential supplied from the quencies and very low gain on undesired fretransformer winding 62 becomes sufficiently high, the grid will permit the flow of current to anode 61.

It will be seen that when no current is flowing in the anode circuit of the A. V. C. valve,v the positive pole of battery 46 is substantially at ground potential. l The bias on grids` 43 and 48 of the second and third stage amplifier valves is ltherefore determined by the potential of the battery 46 and the gain of the amplifier is fixed thereby. However, when current Vflows .in the anode circuit of the A. V. C. valve through the resistors 12, 13 and 14, the potential of the conductor '16 becomes' negative with respect to ground by an amount corresponding to the volt` age drop across the resistor 14, and the bias on the grids Li3 and k48 is thereby rendered more negative to reduce the gain of the amplifier.

Condensers 9| and 93 are of relatively large capacity, .for example of the order of one mic'rofarad, and in conjunction with resistances 12 and 13, constitute a time delay circuit preventing immediate reduction of sensitivity in the amplifier on the arrival of a single impulse of large amplitude, the negative bias on the amplifier valves gradually increasing during the arrival of several successive impulses of high amplitude while the condensers 9| and 93 are being charged, so that noticeable distortion is avoided. Again, these condensers hold the charge thereby im..

parted to them for a considerable period of time,

setting of the resistors 13 and 1 Thus on the arrival of the earlier waves of excessively large amplitude, the grid 68 of the A. .V. C. valve is driven sufliciently positive by the signal energy derived from the voutput transthe rate of discharge being determined bythe 'former secondary 62 to cause a flow of current to the anode 61, thereby charging the condensers 9| and 93 and applying a more negative bias to the grids 43 and 48 of the amplifier valves to reduce the gain of the amplifier. The setting of the resistor 18 determining the bias on the A. V. C. valve should be such that each of the swings of the signals derived from these excessively large waves causes the A. V. C. valve to function and therefore the charging of condensers 9| and 93 continues until such waves have passed. The rst of the reflected waves may be normally too smal1 to cause functioning of the A. V. C. valve but the gain of the amplifier does not immediately return to normal since the charge on condensers 9| and 93 leaks off slowly and hence the bias on the amplifier grids 43 and 48 becomes more positive gradually. The time required for this charge to leak off can be set by appropriate adjustment of resistor 14 to extend substantially for the time during which it is desired to record the reflected waves, or preferably for a lesser time in order to ensure adequate amplification throughout the record, the gain slowly increasing as the strength of the waves is reduced because of the lowering voltage on 9|, producing a chart of reasonably uniform character. In the event of arrival during this period of waves sufficiently large to again-render the A. V. C. valve operative, the charging of condensers 9| and 93 is repeated.

In the prior filed application Serial No. 290,928, hereinbefore mentioned, the threshold value above which the A. V. C. is caused to function remains constant during the taking of a record. Consequently, oneach occurrence of the arrival of seismic waves `of suicient amplitude, the sensitivity of the amplifying system is momentarily reduced, with the result; that substantially increased amplification toward the end of the rec-` ord, which is desirable for reasons hereinbefore pointed out and which would normally occur in the absence of later arriving waves of high amplitude, is prevented by -the repeated charging oi' condensers 9| and 93. I accordingly prefer to provide means, of which one form will now be described, affording a gradual increase of the threshold value with time, so that the amplitude of signals required to cause the A. V. C. to func.- tion is substantially larger toward the end of the -record than nearer the beginning of the record.

Thus I may introduce a condenser |01, in shunt with resistor |08, between the positive terminal of the battery 80 and the ground, in combination with means for gradually charging the condenser |01 during the taking of a record, whereby the condenser charge is added to the voltagedrop across resistor 19 in such manner as vto render more negative the bias applied to grid of the A. V. C valve. For this purpose I have shown acondenser |06, connected in series with variable resistor |0| across condenser |01, the

arrangement being therefore such that any voltage impressed on condenser |06 will gradually be applied to condenser |01 through resistor |0|.

In order to charge the condenser |06, which is of relatively small value, I may provide resistors |02 and `|03 which are arranged in series across 75 the source of cathode voltage for the amplifier valves. A slider on resistor |02 is selectively connected through a relay switch |05 to that side of condenser |08 remote from the ground, the relay being actuated to close the switch in the first instance by a winding H2 which is energized by current owing from the anode 61 of the A. V. C. valve, the switch -being thereafter retained in closed position by a winding ||3 which is energized on closing of switch |09, the latter being similarly closed initially by energization of winding ||2. Resistor |03may be given a sufticiently high value to prevent rapid drainage of the charge impressed on condensers |03 and |01.

It will be appreciated that on the occurrence of a signal in excess of the threshold value, resulting in operationof the A. V. C. valve, and resultant charging of condensers 9| and 93 as herein- `before explained, the relay switch |05 will be operated by energization of the winding l I2, and the switch will thereafter be retained in closed position by the resultant energization of winding 3. On the closing of the switch |05, the voltage drop across part of the resistor |02 is applied to charge the condenser |06, and the charge gradually leaks ofi through resistance |0| to the condenser |01, with the result that the bias voltage applied to the grid 68 by the source 80 is gradually increased in a negative direction as the charge on condenser |01 increases. In other words, beginning with the instant of first operation of the A. V. C. circuit, the threshold value of the A. V. C. valve steadily increases toward the end of the record or until the condenser |01 acquires the potential of the condenser |06, the charging period for condenser |01 being controlled by adjustment of resistor i0 I, and the ultimate voltage impressed thereon being regulated by adjustment of resistor '|02. Consequently, any succeeding operation of the A. V. C. circuit as the result of `re peated arrival of high energy waves must be caused by waves of greater intensity than those required to eiect initial operation of the A. V. C., with the result that the sensitivity of the amplifier in the taking of any given record is generally substantially higher toward the end of the record than if the circuit disclosed in the aforesaid application of Olive S. Petty, having a constant threshold value, were employed.

It will be appreciated that the desired variation in sensitivity of the amplifier with time may be initiated at some other point in the taking of the record. For example, instead of connecting the winding 2 inthe anode circuit of the A. V. C. valve, I may connect the terminals of the winding in shunt or in series with that movable element of the galvanometer which is supplied with current on .the firing of the charge, for example by means of the circuit shown in my prior application, Serial No. 326,947, filed March 30, 1940, which has ripened into Patent No. 2,331,623, dated October 12, 1943. 'In this manner the bias on the grid 'B9 may be increased beginning with the instant of the cap break. Arrangements for effect-- -ing energization of winding ||2 at other desired of the series, with resulting simplification in the circuits of such units.

Having thus described the invention, whatV is claimed as new and-desired to be secured by Letters Patent is:

1. An amplifying system for amplifying a train -of lwaves extending over a period of several seconds and in which the rst part of the train has a general vamplitude level several times greater than that of the remainder and in which the general level vof the remainder gradually decreases, the combination with a seismometer for converting incoming'seismic energy into electrical wave form signals, and means for amplifying and recording such signals, said amplifying means including at least one thermlonic valve amplier, of a thermionic control Valve for applying to a controlgrid of said valve amplifier a more negamore negative toward the end of the period of arrival of the train to effect a corresponding increase in said predetermined amplitude independent of subsequent incoming energy, said last named means acting over the major portion of the period of arrival of the train. l

2. An amplifying system foramplifying a train of waves extending over a period of several seconds and in which the first part of the train has a general amplitude level several times greater than that of the remainder and in which the general level ofthe remainder gradually decreases, the combination with a seismometer for converting incoming seismic energy into electrical wave form signals, and means for amplifying and recording such signals, said amplifying means including at least one thermionic valve amplifier, of a ther'xnionic control valve for applying to a control-grid of said valve amplifier a more negative bias in response to increase in amplitude of in` coming energy above a predetermined amplitude'. means normally biasing the grid of said control Avalve beyond cut-off, means applying to the grid of said control valve a bias derived from the incoming energy of such value as to render the grid potential less negative than said normal bias when said energy exceeds said predetermined amof the train.

3. An amplifying system for amplifying a train of waves extending over a period of several seconds and in which the first part of the train has i a general amplitude level several times greater than that of the remainder and in which the general level ol` the remainder gradually decreases,

the combination with a seismometer for convertv ing incoming seismic energy into electrical wave form signals, and means-for amplifying and recording such signals, said amplifying means including at least one thermionic valve amplifier, of a thermionic control valve for'applying to a control grid of said valve amplifier a more nega-- tive bias in response to increase in amplitude of incoming energy above a predetermined amplitude, means normally biasing the grid of said control valve beyond cut-olf, means applying to the grid of said control valve a bias derived from the incoming energy of such value as to render the 'grid potential less negative than said normal bias when said energy exceeds said` predetermined amplitude, and means, automatically operable in response to the reception of such energy in exce of said predetermined amplitude to gradually render said normal bias` more negative toward the end of the period of arrival of the train to eiect a corresponding increase in said predetermined amplitude independent of subsequent incoming. energy, said last named means acting over the major portion of the period of arrival of the train and including a condenser, devices operatively connected to effect gradual charging of said condenser beginning with the receptie-n of energy vin excess of said predetermined amplitude, and *connectionsv between said condenser and the grid of said control valve for applying the condenser charge to theflatter so as to vaiy with time, and in a negative direction, the no rmal bias on said control valve.

4. An amplifying system for amplifying a train of waves extending over a period of several seconds and in which thefirst part of the train has a general amplitude levelfseveral times greater than that of the remainder and in which the general level of the remainder gradually decreases, the combination with a seismometer for converting incoming seismic energy into electrical wave form signals, and means for amplifying and recording such signals, said amplifying means including at least one thermionic valve amplifier, of a thermionic control valve for applying to a control grid of said valve amplifier a more negative bias in responseto increase in amplitude of incoming energy above a predetermined amplitude, means normally biasing the grid of said control valve beyond cut-off, means applying'to the grid of said control. valve a bias derived from the incoming energy of such value as to render the grid potential less negative than said normal bias when said energy exceeds said predetermined amplitude, and means automatically operable before the'major portion of the Wave train has been received to gradually render said normal lbias more negative toward the end of the pei riod of arrivalof the train to eifect a 'corresponding increase in said predetermined amplitude independent of subsequent incoming energy, said last named means'acting over the major portion ofk the pe'riod of. arrival of the train, and including a source of potential, a condenser, connections between said source and said condenser operable to effect gradual -charging of said condenser by said source, and connections between said condenser and the grid of said control valve for applying the condenser charge to the latter soas to vary with time, and in a vnegative direction, the normal bias on said control valve.

- f JOSEPHUS O. PARR, JR.` 

